Run to the country side. Climb the mountains. Search out a dark field. Look up at night sky.

Imagine two things: first, your feet firmly fastened to Earth. Second, turn it over so ground is up, sky is down. Now hanging from the planet, stop looking at a flat, speckled mat. Instead, peer into the deep vault of heaven. Dive into the sea of stars, each a local neighbor.

Looking at the Milky Way, we see our own galaxy edge-on

Swim farther in. Look down at the bright band cutting across the face of the void. That’s our own Milky Way galaxy seen edge-on. Frisbee flat, billions of stars circle a common center in exuberant promenade. Big? Light, traveling at 186,000 miles (7.5 times around the world) every second, needs 100,000 years to cross the disc. Yet despite its size, we believe we have a good idea how it works.

A spiral galaxy similar to our Milky Way, seen face on. Light takes 100,000 years to get from one edge to the other.

We like to picture space as an immense room filled with billiard balls: stars, 100’s of thousands miles across; planets like Earth, a mere 8,000 miles wide; comets, asteroids and rocks. A melee of spinning bodies, usually orbiting on another, sometimes colliding to ricochet off in new directions. The bigger orbs pull with more gravity, smaller ones less. Complex? Yes, but mechanically comprehendible.

For example, long before we discovered Pluto and sent the New Horizons probe, astronomers agreed the planet Neptune moved oddly. Percival Lowell crunched the numbers in the early 1900’s predicting ‘Planet X’ to be out there, somewhere. In 1930, Clyde Tombaughlooked where the numbers pointed.Voilà, there it was, just as predicted.

If we know a few basics such as, size, mass, speed and direction an object moves, we can figure out what’s going to happen when it gets near something else. That’s how we sent people to the Moon and probes to other planets. Simple really. Just like marbles.

Or, maybe not.

In the 1930’s astronomers started saying, “Wait a minute, something’s not right. Planets in our solar system move as predicted—we did find Pluto, after all. But some stars move too fast.”

Dark matter is invisible, but astronomers added blue shading to this photo of a galaxy cluster to show where dark matter is thought to be

Thirty years later their colleagues, able to see farther away, added, “Galaxies beyond our own don’t have enoughstuff—stars and planets—to spin so fast. It’s like something’s missing.” The puzzled scientist holds his coffee, pauses, scrunches his brow, bites his lip, then looks up not really seeing the ceiling. After a moment he levels his gaze and says, “Yeah, for the way stars and galaxies move, about 80% of the required matter simply isn’t there. We’ve hunted and can’t find anything. Yet some kind of unknown, unique substance is at work out there.”

Dubbed “dark matter,” it has risen to become one of the greatest mysteries in astrophysics. Dark matter neither emits, absorbs, nor reflects light. We can’t see it, hear it, feel it, taste it, smell it, box it, weigh it or measure it. Its effect on things around it offers the only clue that it does, in fact, exist. Less detectable than wind blowing leaves, but far more powerful, it challenges how we understand physical reality. And, of course, not everyone agrees that it exists after all.

Just like hope.

Hope seems made of nothing, mere wishful thinking, imagination or blatant denial. A word used only to blow smoke about what we pretend will happen if everything, by chance, works out. No one sees it, hears it, feels it, tastes it, smells it, boxes it, weighs it or measures it. Like dark matter, it submits to no physical test and not everyone agrees that it even exists as anything but a figure of speech.

Hope, like super rocket fuel, propels us farther than we can reach, sustains us longer than we can last, strengthens us to carry more than we can lift